To prepare the pork, coat the inside of a slow cooker with nonstick cooking spray. Place the onion on the bottom of the cooker.

In a small bowl, stir together the black pepper, garlic granules, salt, chili powder and cayenne pepper. Rub the mixture all over the pork roast. Place it on top of the onion in the slow cooker and add enough water or beer to fill the cooker two-thirds full. Cover and cook on low according to the manufacturer's instructions for 8 to 12 hours or overnight. 

Remove the roast from the slow cooker. Discard the bone and fat, as well as any water, onion and grease remaining in the pot. When the meat is cool enough to handle, use a fork or your fingers to shred it. Return the pork to the slow cooker. Stir in 2 cups of the barbecue sauce (recipe below), cover and cook on medium-high for 1 to 3 hours.

Serve the pulled pork on sourdough buns doused with Denise's secret sauce (recipe below).

To prepare the barbecue sauce, in a large saucepan, combine the ketchup, vinegar, brown sugar, Worcestershire, lemon juice and horseradish. Bring to a soft boil and boil for 6 minutes. The sauce can be made in advance and refrigerated until ready to use.

To prepare the secret sauce, in a bowl, stir together the salsa verde, mayonnaise, ketchup and chili powder. Transfer the sauce to a squeeze bottle and refrigerate.